1. Field of the Invention
The invention relates generally to programmable logic devices, and more particularly to a programmable logic device having an on-chip analog functional block, such as a phase lock loop.
2. Description of Related Art
Programmable logic devices such as the programmable array logic (PAL) device offer digital designers a flexible and cost-effective implementation for complex logic circuits. PAL is a trademark of Monolithic Memories, Inc. A typical PAL includes a fuse-programmable or electrically erasable programmable array of AND gates, and a fixed array of OR gates. In some programmable logic circuits, the OR array is itself programmable. The outputs of the combinatorial logic arrays of programmable logic circuits may be coupled directly to an I/O pin, or input to clockable registers. In many devices, registered outputs are fed back to the inputs of the combinatorial array. Some programmable logic circuits also include a clockable input synchronizing register located between an input pin and one of the inputs to the combinatorial array.
Programmable logic circuits having clockable registers are ideal for use as state machines or, as they are sometimes called, sequencers. A state machine includes a number of registers, which store the current state of the machine, and a combinatorial logic array, for providing the next state to be stored in the registers. Inputs to the combinatorial array are provided both from external pins and from previous outputs of the state registers fed back to inputs. In all programmable logic devices currently available, the clock signal which is used to clock the various registers is supplied from outside the chip, either directly or via the logic array.
The programmable logic device has given the digital designer a means of reducing circuit size through higher integration, ease of design and documentation by software specification, and security of keeping the design proprietary. There is currently no equivalent to the programmable logic circuit in the analog realm. The semi-custom "analog array" is bringing large scale integration to the analog designer, but the approach is still very expensive. Part of the reason for the lag between the analog and digital worlds is that designers typically try to accomplish analog functions by performing an analog-to-digital conversion as soon as possible, implementing the solution in the digital realm and then performing a digital-to-analog conversion as the final step.
All the programmable logic devices currently on the market are entirely digital in nature, with no analog components included. Even at the board level it has been unusual for system designers to incorporate digital programmability in an analog functional block since the design engineer must be confident designing with both analog and digital parts. There are currently no devices on the market, as far as Applicants are aware, where a circuit has both analog and digital components working together to accomplish a programmable function. Fixed function devices exist (such as the Am8151 color palette) which incorporate both analog and digital components, but until now all of these devices have lacked the flexibility of programmability.